1. Field of the Invention
The present invention relates to: a light source type discriminating method; an image forming method; a method and apparatus for estimating a light source energy distribution, which are adapted to determine the type of light source illuminating a subject; and an exposure amount determining method using the same. More particularly, the present invention relates to: a light source type discriminating method which makes it possible to estimate not only the color temperature of a target light source but also the type of fluorescent lamp registered in advance; an image forming method which makes it possible to estimate the type of photographic light source among various light sources including artificial light sources (12 types of fluorescent lamps defined by the CIE, for instance) and to perform appropriate image processing based on a result of this estimation; a method and apparatus for estimating a light source energy distribution, which are adapted to estimate the light source energy distribution of an illumination light source on the basis of information obtained through image pickup of a subject illuminated by the light source; and an exposure amount determining method.
2. Description of the Related Art
The printing exposure amount required when a photographic film image is printed on a duplicating sensitive material, such as photographic paper, is determined according to the quantity of light received by the photographic film (hereinafter also simply referred to as the “film”) from a subject at the time of photographing, and the printing exposure amount differs from frame to frame. To obtain a print with favorable color reproductivity, it is required to correct the printing exposure amount in accordance with photographing conditions. To this end, generally, a gray balance is determined by measuring an integral transmission density for each of red (R) light, green (G) light, and blue (B) light using a photometer, which is provided with a color-separation filter formed of dye filters or vapor deposition filters, and determining an exposure amount required when a color image is reproduced on a duplicating sensitive material from an original color image for each of the R light, G light, and B light.
However, information on photographic light quality may vary depending on color failure in a background or the like, development conditions, or the like. This makes it impossible to accurately estimate the light quality, and hence the color reproductivity may become poor depending on variations in the quality of light illuminating a subject. This is because it is impossible to make a judgment as to which portions are gray on the film.
The most effective method used for detecting the gray portions on the film is to estimate the color temperature of a photographic light source.
As to this method, for instance, one of the inventors of the present invention proposes, in U.S. Pat. No. 5,636,143, a method in which the color temperature and spectral energy distribution of a photographic light source are estimated from a signal of an image recorded under given photographing conditions.
This method is aimed at estimating the color temperature of a light source used at the time of photographing a subject, and more specifically at estimating the color temperature of the light source from information on a photographed image. In brief, the principle of this method is to estimate the light source color temperature from a balance between R (red), G (green), and blue (B). For instance, G is set as a base point and the light source color temperature is estimated from a balance between R and B.
That is, information on a photographed image is a result of multiplication of the spectral energy distribution of a light source and the spectral reflectance distribution of a subject. Therefore, in this method, a process where the color temperature of the light source is assumed and the spectral reflectance of the subject is estimated based on the assumed color temperature is carried out for each of various types of light sources.
If such waveform prediction is performed on data of an image actually photographed, however, unreasonable data is obtained depending on the assumed light source. That is, there is a case where an abnormal value that is impossible under normal circumstances, such as a reflectance exceeding 100% or a negative value, is outputted.
When such an abnormal value is outputted, a process for adding the abnormal value as a penalty is performed for every pixel in an image (or certain pixels extracted from the image). FIG. 9 is an example of a graph obtained by setting the color temperature on a horizontal axis and plotting the value of the penalty described above.
In FIG. 9, it can be said that a light source having a color temperature corresponding to a point with the lowest value of the penalty has the highest possibility of being an estimation target light source that was actually used at the time of photographing.
Up to this point, we have provided an overview of the technique disclosed in U.S. Pat. No. 5,636,143, that is, a method of estimating the color temperature of a black body radiation light source by detecting reflectance abnormalities using three R, G, and B sensors.
This method makes it possible to appropriately estimate the color temperature of a black body radiation light source such as sunlight or a light source like an incandescent lamp equivalent to the sunlight. Therefore, it can be said that this method is a photographic light source estimating method that is extremely effective when a black body radiation light source is the photographic light source.
Even in this method, however, there is still a problem stemming from a fact that photo taking is performed indoors as well as outdoors. In the case of the indoor photo taking, photographs are taken under a fluorescent lamp in many cases and, as is well known, the fluorescent lamp differs from other general light sources that are black body radiation light sources. This means that it is impossible to properly estimate the type of photographic light source even by using the aforementioned method based on the color temperature detection or the method proposed by the inventor of the present invention for estimating the color temperature of a black body radiation light source.
In order to solve this problem, various techniques have been proposed up to now, although there is left unsolved the problem that when the photographic light source includes various fluorescent lamps as well as black body radiation light sources, it is impossible to sufficiently discriminate between the fluorescent lamps and the black body radiation light sources.